Furnaces for solid fuel

ABSTRACT

A FURNACE FOR BURNING SOLID FUELS SUCH AS BITUMINOUS COALS COMPRISES A GRATE, A FUEL INLET SHAFT WHCH COMMUNICATES ONLY AT ITS LOWER END WITH A COMBUSTION CHAMBER ABOVE THE GRATE BY MEANS OF A RESTRICTED OPENING BENEATH A SOLID TRANSVERSE WALL, THE LOWER END PORTION OF THE FUEL INLET SHAFT FORMING A DEGASSING CHAMBER. A GAS DUCT IS PROVIDED FOR CONVEYING GAS   FROM THE DEGASSING CHAMBER FOR DISTRIBUTION BENEATH A REARWARD SECTION OF THE GRATE. IN ACCORDANCE WITH THE INVENTION, THE BOTTOM END OF THE DEGASSING CHAMBER PORTION OF THE INLET SHAFT IS CLOSED.

United States Patent 1113 5 7 [72] Inventor Viking Valentin Demar [56] References Cited g zn sl UNITED STATES PATENTS 1 PP 3 413927 10/1889 Blanchard 110/31 ggff 9332139 2 915,034 3/1909 Lucke 110/31 i [73] Assignee culoscarAknnderEkman 1,149,056 8/1915 Houston 110/31 Djursholm, Sweden Primary Examiner-Edward G. Favors [32] Priority May 2,1968 Attorney-Cushman, Darby and Cushman [33] Great Britain ABSTRACT: A furnace for burning solid fuels such as bituminous coals comprises a grate, a fuel inlet shaft' which communicates only at its lower end with a combustion chamber FURNACES FOR SOLID FUEL above the grate by means of a restricted opening beneath a 21 Claims 5 Drawing solid transverse wall, the lower end portion of the fuel inlet [52] U.S. Cl 110/31, shaft forming a degassing chamber. A gas duct is provided for 48/76 conveying gas from the degassing chamber for distribution [51] Int. Cl Cl0b 31/02 beneath a rearward section of the grate. In accordance with [50] Field of Search 110/31; the invention, the bottom end of the degassing chamber por- 48/76 tion of the inlet shaft is closed.

FURNACES son soun FUEL This invention relates to furnaces for burning solid fuels 1 such as bituminous coals smokelessly, the furnaces being of the kind in which under normal working conditions gases and volatile matter including tar (hereinafter referred to generically as gas) from the green coal are released and mixed with air and the mixture is thereafler raised to a sufficiently high temperature to ignite by being passed through the incandescent fuel-bed at the rear of the grate.

An example of such a furnace is described in British Pat. specification No. 1 ,094,44 1

With a view to providing a furnace of simple and relatively cheap construction while ensuring efficient and substantially smokeless combustion while ensuring efficient and substantially smokeless combustion with a high percentage of CO and a minimum of residual unburnt combustible matter in the ashes, the principal object of the present invention is to ensure controlled and effective degassing.

According to the present invention there is provided a furnace for burning solid fuels having a gas content as defined above, comprising a grate, a fuel inlet shaft which communicates only at its lower end with a combustion chamber above the grate by means of a restricted opening beneath a solid transverse wall, the lower end portion of the fuel inlet shaft forming a degassing chamber, and a gas duct for conveying gases from the degassing chamber for distribution beneath a rearward section of the grate, wherein the bottom end of the degassing chamber portion of the inlet shaft is closed.

With this arrangement a well-defined boundary zone is created between the burning coal on the grate and the green coal in the inlet shaft.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a longitudinal sectional view of the furnace,

FIG. 2 is a perspective view of some of the more important parts of the furnace, with some portions broken away,

FIG. 3 shows a fragment of a cross section on the line 3-3 of FIG. 2,

FIG. 4 is an elevational view of part of the furnace, and

FIG. 5 is a longitudinal vertical sectional view of a hopper which may be used for feeding coal to the furnace shown in FIGS. 1 to 4.

Coal is fed to the furnace shown in FIGS. 1 to 4 from the outlet of a hopper and descends under gravity down a coal inlet shaft 11. The bottom of the shaft 11 is formed by a dead plate '12 which extends over the width of a forward extension of a horizontal grate 13 between sidewalls 14 of the furnace.

The front wall of the coal inlet shaft 11 is formed by a coal guide 15 which is slightly inclined to the vertical to allow the descending coal to swell as it is degassed. The rear wall of the shaft is formed by a transverse refractory arch block 16 disposed above the grate 13 so as to define a restricted opening 17 connecting the coal inlet shaft 11 with a combustion chamber 18 situated beneath a top refractory arch formed by a plate 19.

The space beneath the grate I3 is divided by transverse partitions 20 and 21 into an air inlet chamber 22 and a gas distributing chamber 23.

All the air required for combustion enters the furnace through openings in the front plate of the air inlet chamber 22 as indicated by the arrow 24. Natural chimney draught, forced draught or induced draught may be employed as desired.

The fuel on the grate 13 forms a burning fuel-bed 25 which slopes away from the restricted opening 17 to the rear end of the grate 13 where the ash and any clinker which may have formed drop on to an ash-removing screw-conveyor 26 which extends across the full width of the furnace. From this point it may be removed in any appropriate manner depending upon the location of the furnace. In the embodiment illustrated, it is taken out through one side wall 14.

The grate is formed by a plurality of reciprocable parallel grate bars 27 of generally triangular or tapering cross section as shown in FIGS. 2 and 3. The grate bars 27 have three sets of laterally projecting abutments, namely: abutments 28 at their forward ends, abutments 29 at their rear ends and abutments 30 at an intermediate region above the transverse partition 20. The abutments 28, 29, 30 on each bar 27 abut the corresponding abutments on the adjacent bar so that the portions ofeach grate bar 27 between the abutments 28 and 30 and between the abutments 30 and 29 are spaced from the corresponding portions of the adjacent bars. In this manner a primary air flow channel 31 is formed between the air inlet chamber 22 and the fuel-bed 25. Similarly a secondary gas flow channel 32 is formed between the gas distributing chamber 23 and the rearend of the fuel-bed 25.

In order to prevent undue escape of air adjacent the sidewalls I4, where the resistance to air flow is less than elsewhere in the fuel-bed, it is preferred that the two outermost grate bars should be of rectangular cross section as shown at 33 in FIG. 2. In this way a more uniform air distribution across the fuel-bed is maintained.

All the grate bars 27 and 33 are slidably supported at their forward ends on a transverse bar 34 and at their rear ends by the top of the rear wall partition 21 of the gas distributing chamber 23.

The coal guide 15 forms a boundary between the coal inlet shaft 11 and a gas receiving chamber 35 at the front of the furnace. The lower part of the coal inlet shaft 11 communicates with the gas receiving chamber 35 through gaps between downwardly projecting teeth 36 in the lower part of the guide 15 and a horizontal opening 37 beneath the teeth 36.

The gas receiving chamber 35 is connected to the gas distribution chamber 23 by two gas ducts 38 and 39 (FIG. 4). The entry to the gas duct 38 is shown at 40 in FIG. 1 and its outlet at 41. It is preferred that the gas ducts have a uniform circular cross section throughout their length in order to minimize their frictional resistance.

A reciprocating mechanism for the grate bars 27 comprises a rotationally driven shaft 42 carrying radial arms 43, one for each grate bar. Each grate bar has a cam surface 44 engaged by a roller 45 carried by the corresponding radial arm 43. The radial arms 43 are divided alternately into two angularly spaced groups as shown in FIG. 1. As the shaft 42 rotates in the clockwise direction (FIG. 1) one group of corresponding grate bars is retracted to the forward end position before the other group. However a crossbar 46 interconnecting the two outermost bars 33 ensures that all the grate bars are then returned to the rear position shown in FIG. 1 simultaneously as a result of engagement of the cross bar 46 with abutments 47 on the grate bars.

The top surfaces of the grate bars may slope as shown at 48. The relative movement between adjacent grate bars will then cause the top surface of the grate to undulate, thereby opening up the fuel-bed and discouraging the formation of clinker.

The shaft 42 is driven by an electric motor 49 through a chain drive 50. In order to adjust the speed of the grate and thus the throughput of the furnace, the voltage supplied to the motor 49 may be varied in order to control the speed of the motor.

The ash removal screw conveyor 26 is driven by the same motor 49 through a chain drive 51 (FIG. 4). Any adjustment.

made through this motor to the speed of the grate therefore correspondingly adjusts the speed of the drive to the ash removal screw conveyor 26.

In order to feed the fuel to the grate a reciprocating pusher bar 52 projects rearwardly over the dead plate 12 with a running clearance forming effectively a seal between the pusher bar and the lower edge 53 of the bottom of the gas receiving chamber 35. The pusher bar 52 is secured by bolts 54 (FIG. 2) to the outermost grate bars 33 so as to move with them.

In operation, the shafi 11 is filled with coal and the burning fuel on the grate 13 forms a bed of the general shape indicated at 25.

The draught above the fuel-bed 25 in the combustion chamber 18 causes air to be drawn through the inlet channel 31 in the direction of the arrow 55. To direct this airflow the upper portion of the partition 20 and the front surfaces of the abutments 30 are inclined upwards and rearwards and are aligned to guide part of the airflow to follow the arrow 55.

A smaller airflow is diverted in the direction of the arrow 56 to pass through small channels 57 provided between the tops of the abutments 30 and the top surface of the grate 13 as shown in FIGS. 1, 2 and 3 so as to cool the grate bars in this region. However, the tops of the abutments 30 have a length, measured longitudinally of the grate bars, at least equal to the reciprocatory stroke of the grate so that in all relative positions of the grate bars an effective seal is provided between the channels 31 and 32 other than in the region of the channels 57.

The position of the rear edge 58 of the dead plate 12 almost directly underneath the inlet edge 59 of the opening 17 and the relatively high step (preferably between 25 percent and l percent of the height of the opening 17) between the bottom of the block 16 and the top arch l9 ensures that the air drawn through the inlet 24 in the direction of the arrow 55 has a well defined boundary which in turn defines a boundary zone between the green coal in the shaft 11 and the entrance to the opening 17 on one side and the burning fuel in the outlet portion of the opening 17 and in the combustion chamber 18 on the other side. This boundary zone is indicated by a broken line 60 in FIG. 1.

As the gas distributing chamber 23 is sealed by the partitions 20 and 21 and the abutments 29 and 30, a suction created by the chimney or induced draught fan is created in this chamber due to the draught above the fuel-bed 25. The gas distributing chamber 23 is connected to the gas receiving chamber 35 and this suction is of the same magnitude as that existing above the fuel-bed 25 less the resistance presented by the coal in the inlet shaft 11, the gas ducts 38 and 39, the grate l3 and the fuel-bed on the grate. In order to minimize this resistance the shaft 11 is made relatively narrow by the coalguide and the gas ducts are short and have a uniform circu lar cross section throughout their length.

Due to the suction in the gas receiving chamber 35, a hot gaseous stream is drawn towards it from the burning fuel behind the boundary zone 60. This stream causes the green coal above the dead plate 12 to give off a portion of its volatile matter as gas. The suction in the gas receiving chamber also causes some of the air passing through the primary grate channel 31 adjacent the dead plate 12 to be drawn towards the chamber 35. The mixture of gas and this air is thus drawn into the gas receiving chamber 35. This mixture has a very low temperature (for example 50 C.) and is continuously evacuated through the gas ducts 38 and 39 to the gas distributing chamber 23 and is then passed up in the direction of the arrow 61 through the channel 32 and the burning fuel-bed on the grate 13 in a very hot region where the mixture is raised to ignition temperature (700 to 800 C.) and is ignited above the hot fuel-bed. This causes a high combustion temperature which results in minimal unbumt carbon in the ash residue which is discharged over the rear end of the grate.

Any tendency of the suction in the gas receiving chamber 35 to draw the boundary zone 60 forwards into the coal inlet shaft 11 is overcome by the action of the reciprocating pusher moving the green coal in the direction towards the combustion chamber.

As a result of the more direct path for gas through the top of the fuel bed directly into the combustion chamber immediately beneath the block 16 as compared with the path through ducts 38 and 39, a portion of the gas together with some air will follow paths indicated by the arrows 62 in FIG. 1 to sweep under the hot base .of the refractory block 16. ln order to ensure heating of this portion of the gases to their ignition temperature, the lower portion of the block 16 should extend longitudinally of the furnace for a sufficient distance which in the embodiment shown is almost the same as the length of the dead plate 12. The transverse block is made of refractory material of sufficiently low thermal conductivity to maintain a sufficiently high temperature at its lower rear edge 63 to ignite the gases and at the same time to limit the heat conducted from the combustion chamber to the inlet shaft 11 and thereby to limit premature gasitication of the coal in the shaft.

Since there is only one air inlet for the entire air required for combustion and furthermore since the fuel-bed on the grate is always of substantially the same height and therefore offers the same resistance to the passage of air and gas through it, it is very easy to regulate the air supply for maximum efficiency of combustion. All that is necessary is for the airflow through the air inlet chamber 22 to be regulated in accordance with the speed of the coal feed. in the case of natural draught, the airflow may be controlled by a damper while with a forced or induced draught the driving motor for the fan may be under the same control as the motor 49.

The furnace may be mounted in a boiler having its front wall located in a position shown by the broken lines 64. The top plate 19 and the side plates 14 will then be made of a refractory material having relatively high thermal conductivity.

Where the coal is to be supplied under gravity, it is essential to avoid pressure on the green coal in the inlet shaft 11. his accordingly an object of a further aspect of the invention to provide a feeding device which will feed coal from a gravity supply to the furnace inlet as required by the furnace without transmitting the pressure in the coal on the inlet side of the feeding device to the furnace inlet.

According to this aspect of the invention there is provided a feeding device for feeding coal from a gravity supply to its outlet, in which a weir defines a free surface for coal descending under gravity through the device and only the coal in the layer adjacent to this free surface has access to the outlet of the device.

FIG. 5 shows a feeding device in the form of a hopper base for feeding coal to the inlet shaft 11 without subjecting the coal in the shaft to the weight of the coal in the hopper above the hopper base.

The hopper base shown in FIG. 5 forms the lower end of a vertically extending coal hopper 101 of rectangular section. Normally the coal in the'hopper forms an air seal for the outlet 10 but a detachable cover 102 may be provided if necessary. At its lower end, the rear wall 103 of the hopper is inclined downwardly and forwardly at an angle of not less than 45 to the horizontal. A baffle plate 104 is slidably mounted on the wall 103 and carries a screw-threaded rod 105 extending through a slot in the inclined wall 103. On this rod 105 is screwed a correspondingly internally screw-threaded knob 106 by means of which the baffle plate 104 can be fixed in any desired position. I

The lower edge 107 of the baffle plate 104 forms a weir for the coal descending from the hopper 101. Since coal has an angle of repose of about 45, the baffle plate edge 107 'determines the position of a coal surface 108 which slopes at about 45 to the horizontal. Coal from the hopper 101 cannot reach the space above and to the left of the surface 108 in the FIG. irrespective of the quantity of coal in the hopper 101.

The baffle plate 104 can be moved to a lower position in which its edge 107 is advanced to the position shown at 109 and thereby moves the coal surface to the position shown at 114. in this position, the coal surface at 114 lies to the right of the edge 110 of the outlet 10. Accordingly no coal from the hopper 101 can enter the furnace through the outlet 10. In order to feed coal to the outlet 10, the baffle plate 104 must be moved upwards until its lower edge 107 defines the free surface 108 shown in the F16. Coal from the surface 108 is then able to slide over the edge 110 of the coal outlet 10 until the latter becomes completely filled with coal. As soon as this occurs the lower end of the surface layer of coal will be supported so that no further coal can discharge down the face 108 until the coal level in the outlet 10 drops.

instead of relying on the free discharge of coal down the face 108, an agitator 110 may be employed. The agitator shown in FIG. 5 consists of a shaft 111 carrying two short wings 112. Conveniently the shaft 111 is connected by a chain and sprocket drive to the shaft 42 of the furnace but may of course be driven in any other appropriate manner. The rotating wings 112 skim the surface layer of the coal and cause it to discharge through the outlet 10. The agitator may be arranged to feed the coal towards the outlet at a slightly greater rate than the rate of consumption in the furnace. Coal will then build up to the position indicated at 118. No further coal can be fed by the agitator until the coal level in the outlet 10 drops.

If it is desired at any time to cut off the supply of coal from the hopper to the outlet 10, all that is necessary is to move the baffle plate 104 downwards a short distance so as to move the coal surface 108 towards the position 114 until the coal is beyond the reach of the wings 112 and the lower end of the coal surface is supported on the bottom wall 119 of the hopper at a position spaced from the edge 110. The force required to move the bafile plate 104 through this short distance is relatively small compared with the force which would be required to drive it fully through the coal into contact with the opposite wall 120.

I claim:

1. A furnace for burning solid fuels having a gas content comprising: a grate having forward and rearward sections; means including side walls defining a combustion chamber above said grate; a depending fuel inlet shaft having a lower end portion disposed above the forward section of said grate and forming a restricted opening between said fuel inlet shaft and said combustion chamber, said means including a solid transverse plate, said wall being disposed above said plate and defining the upper surface of said restricted opening and said plate closing the lower end of said degassing chamber and defining the lower surface of said restricted opening; and duct means for conveying gas from said degassing chamber and for distributing the gas below said rearward section of said grate.

2. A furnace for burning solid fuels having a gas content comprising: a grate having forward and rearward sections; said grate including .a plurality of substantially horizontally disposed bars, means for imparting reciprocatory motion to said bars; means including side walls defining a combustion chamber above said grate; a depending fuel inlet shaft having a lower end portion disposed above the forward section of said grate and fonning a fuel degauing chamber; means forming a restricted opening between said fuel inlet shaft and said means including a solid transverse plate, said wall being disposed above said plate and defining the upper surface of said restricted opening and said plate closing the lower end of said degassing chamber and defining the lower surface of said restricted opening; and duct means for conveying gas from said degassing chamber and for distributing the gas below said rearward section of said grate.

3. A furnace according to claim 2, wherein the edge of said transverse plate forming the bottom of the said degassing chamber is located substantially directly below the upper front edge of the restricted opening.

4. A furnace according to claim 2, wherein the longitudinal extent of the restricted opening is approximately the same as the longitudinal extent of the bottom of the degassing chamber.

5. A furnace according to claim 2, wherein the restricted opening is formed by a refractory block extending between the side walls of the furnace to define a substantially constant height for the restricted opening throughout its length between its inlet and outlet.

6. A furnace according to claim 5, wherein the top of the combustion chamber is formed by a second refractory block extending between the side walls of the furnace above the grate, the height of the combustion chamber being substantially constant along its length and being greater than the height of the restricted opening, being preferably more than 25 percent but less than percent greater than the height of the restricted opening.

7. A furnace according to claim 6, wherein the block defining the restricted opening is made of a refractory material of relatively low thermal conductivity while the roof and the side walls of the furnace are formed of material having a relatively high thermal conductivity.

8. A furnace according to claim 2, wherein the fuel inlet shaft is separated from a gas-receiving chamber by an apertured guide arranged to limit the forward extent to the coal in the degassing chamber to within a predetermined distance from the rear edge of the degassing chamber bottom while permitting gas to pass into the gas-receiving chamber and wherein the gas duct leads from the gas-receiving chamber.

9. A furnace according to claim 2, wherein a reciprocatory pusher at the front edge of the degassing chamber bottom serves to urge the fuel towards the grate.

10. A furnace according to claim 2, wherein the grate is extended forwardly beneath the degassing chamber bottom which is formed by said transverse plate extending over a forward extension of the grate.

l l. A furnace according to claim 10, wherein the rear edge of the transverse plate forms an abutment for the fuel-bed on the grate as the grate is retracted to its forward end position.

12. A furnace according to claim 11, wherein the pusher is secured for movement with a number of the grate bars.

13. A furnace according to claim 11, and including driving A means for retracting some bars at a different time to other bars and for advancing all the bars simultaneously towards the rear of the furnace.

14. A furnace according to claim 13, wherein adjacent grate bars define primary and secondary through-flow channels defined by lateral abutments on the bars which make sealing contact with corresponding abutments on the adjacent bars, the abutments separating the primary and secondary throughflow channels being located above the front wall of a gas distributing chamber beneath the grate, the primary throughflow channel extending between the said front wall and the rear edge of the bottom of the degassing chamber, and the gas duct opens into the gas distributing chamber.

15. A furnace according to claim 14, wherein the longituv 19. A furnace according to claim 18, wherein the longitudinal extent of the abutments separating the primary and secondary channels is not less than the stroke of movement of the grate bars.

20. A furnace according to claim 19, wherein the forward ends of the grate bars are formed with cam surfaces cooperating with cam rollers carried on radial arms mounted on a rotatable driving shaft for imparting reciprocatory movement to the individual grate bars.

21. A furnace according to claim 20, wherein a portion of the top surfaces of the grate bars is inclined downwardly towards the rear of the furnace. 

